The invention relates to a fuel metering system for feeding fuel from a storage tank to the cylinders of a combustion engine, comprising a fuel supply unit and a distributing device comprising at least one rotor, with the fuel supply unit continuously supplying fuel from the storage tank to the distributing device, and with the distributing device, depending on the angle of rotation of the rotor, sequentially feeding fuel to the cylinders of the combustion engine while determining for each cylinder during what period fuel is injected at the cylinder.
Such a system is known from, for instance, German Offenlegungsschrift 2921766. According to the German Offenlegungsschrift, the object contemplated is to provide a system in which the use of electromagnetic (on/off) injectors is avoided. This is realized by a rotor/stator combination, with the rotor being driven by a stepping motor. The angular displacement of the rotor is here time-controlled. The rotor is triggered by a reference on, for instance, a camshaft. After being triggered, the rotor rotates through a predetermined angle under the control of the stepping motor, wholly independently of the crankshaft and camshaft. Thereafter the rotor is stationary for a predetermined period. The period during which the rotor is stationary in an open position is dependent on the load of the engine and not directly on the speed of the engine, i.e., independent of the speed of rotation of the crankshaft and camshaft and hence independent of the instantaneous angle of rotation of the crankshaft and camshaft. During the period when the rotor stands still, fuel is fed to one of the cylinders, or fuel is fed to none of the cylinders. Here, as with systems in which injectors are used with a pulse width control, the fuel supply is switched on and off, respectively, under time control. Such a control of the fuel flow by variation of the open and/or closed time of a rotating valve with a discrete number of stable angular positions, is in practice difficult to realize, if at all. Stepping motors have a limited speed and therefore are generally not suited for such a control. The rotor/stator combination needs time to rotate through the closed position. The longer the mechanical valve (at a particular speed of the engine and the rotor) is in the open position, the more time to move through the closed position is lost. Conversely, the maximum time left for the open position is the time of the period minus the minimum closed time.
Further, the known rotor/stator combination involves a minimum open time: the time needed to move through the open position as fast as possible. That is also a disadvantage of that mechanical metering device over electromagnetic injectors, because the minimum pulse width thereof is zero. The control range of the known quantity control according to the Offenlegungsschrift is therefore by definition smaller than that of an electromagnetic one with pulse width control.
If several cylinders are provided with fuel by one mechanical distributor, an additional problem arises. The rotor rotates (in the case of a four-stroke engine) at half the crankshaft frequency. When the rotor must deliver fuel for all cylinders in one revolution, the frequency of the fuel pulses increases with the number of cylinders. The number of angular positions thereby increases proportionally to the number of cylinders. So at a given speed of the engine (and the rotor) the step time decreases proportionally. Or, at a given step time of the stepping motor, the maximum attainable speed of the rotor decreases proportionally.
The minimum open time is one stepping period. Without adjustments of the fuel pressure, this results in the minimum quantity of fuel being delivered even at zero load (no couple, so no fuel needed). If that quantity is made slight by reducing the passage, this in turn leads directly to unacceptable consequences for full load at high speeds. In that case the rotor cannot remain long in the opened position to deliver the required quantity of fuel because the available period and the time loss in the closed position do not allow this. Without additional measures, therefore, a mechanical metering and distributing device driven directly by a stepping motor cannot work. The control range is too small. Invariably, a certain minimum quantity of fuel is delivered, so that either too much fuel is metered at low engine loads or too little fuel is metered at high loads and high speeds. Moreover, the maximum attainable speed is limited by the stepping motor and the number of discrete angular positions.
According to the Offenlegungsschrift, the fuel flow can be increased by shifting the rotor, in such a manner that the rotor partly clears a number of outlets of the stator. In that case, fuel flows continuously to these outlets and continuous injection is involved.
Summarizing, it can be stated that the system according to the German Offenlegungsschrift entails the following problems:
The rotor position is not instantaneously dependent on the angle of rotation of the crankshaft or camshaft. This has as a disadvantage that injection within a certain angle of the crankshaft or camshaft is not guaranteed. PA1 The relation between the rotor position and the crankshaft position is discontinuous. The rotor has a fixed and limited number of angular positions. The transmission ratio is therefore a "stepped function". The consequence is that the control range in angular positions is zero, while the resolving power is also zero. PA1 The only variable that is available is time. The control range in time is considerably limited by the stepping motor, in that all cylinders are provided with fuel by one rotor and by the fact that many closed positions are present. PA1 The system cannot work correctly under all operating conditions of a combustion engine.